It is known from PCT/GB 2005/003300 and PCT/GB 2010/000798 to form a pump with a housing and a rotor rotatably received in an interior surface of the housing. The housing has an inlet and an outlet and the rotor has a housing engaging surface that co-operates and seals with the interior surface of the housing. The rotor has at least one shaped surface radially inwardly of the housing-engaging surface and forming with the interior surface of the housing a chamber for conveying fluid from the inlet to the outlet on rotation of the rotor. A seal is provided between the outlet and the inlet to engage the shaped surface to prevent the passage of fluid from the outlet to the inlet.
In the pump of PCT/GB2005/003300 and PCT/2010/000798 the surfaces have a shape formed by the intersection with the rotor of an imaginary cylinder having an axis normal to the axis of the rotor. This produces a surface that is concavely curved in planes including the axis of the rotor. This defines the size of the chamber formed by the surface with the housing.
In the prior art, such a shape of surface has an abrupt change in profile where the edge of the surface meets the interior surface of the housing. This limits the maximum rotational speed as, owing to its inherent flexibility, the seal cannot follow the abrupt change of a profile, as is necessary to provide a continuous seal on fast rotations, and the seal is subject to more wear from abrasion caused by the sharp edge which is inherent in an abrupt change in profile.